His understanding of Pokémon could deepen further. Since he now has to design challenges for trainers, he would study Pokémon types and abilities more intensively. He might discover new strategies and combinations that he hadn't thought of during his regular journey. This could also make him more respected in the Pokémon community as a knowledgeable gym leader.
Ash's confidence would grow in a different way. While he was always confident in his battles as a trainer, as a gym leader, he has to project an air of authority. He would learn to handle defeat gracefully when a trainer manages to beat him. This would show his maturity and growth as a character, and also inspire the trainers who defeat him to keep striving for more in their Pokémon journey.
One idea could be that Ash uses his unique battle style which he has developed over his journey to create a really challenging gym. He might include Pokémon from all the regions he has visited in his gym battles, making it a diverse and exciting experience for trainers. For example, he could have Pikachu lead the charge as always, but also include some of his less common Pokémon like Heracross or Gliscor.
Often, it starts with Ash having an internal struggle or realization about his gender identity. He might notice that he doesn't feel like he fits the traditional male role he has been in. Then, he begins to explore this new side of himself, perhaps trying out different appearances or behaviors that are more in line with his true self.
Ash may become more confident. Capturing Mewtwo is no easy feat, and if he manages it, it would boost his self - assurance. He might also become more empathetic as Mewtwo has a complex personality. Understanding Mewtwo could make Ash better at relating to other Pokémon with similar issues.
Ash develops by learning new strategies. In Pokemon Conquest, battles are different, so he has to adapt, which makes him more versatile. He also develops his relationships with new Pokemon, which broadens his understanding of different Pokemon types.
Ash often becomes more confident in these fanfictions. He learns new ways to communicate with his Pokemon during contests, which makes him a better trainer.
In this fanfiction, Ash's character development might involve him becoming more empathetic. He could meet Pokemon or characters in the Lunar region who are in difficult situations. For example, if there are Pokemon being exploited in the Lunar region, Ash would step in to help. This would make him grow as a person and a trainer. Also, his understanding of different types of Pokemon and their needs could expand, since the Lunar Pokemon might have different requirements compared to the ones he's used to. He might also develop new skills in communication, if the Lunar region has its own way of communicating with Pokemon.
Often, in 'pokemon ash and harem fanfic', Ash becomes more confident. With the support of the characters in his harem, he might take on tougher challenges. For example, if a girl in the harem is really good at a certain type of Pokémon battle, Ash could learn from her and become better at that too.
He could develop more patience. Since rare pokemon are often hard to find and catch, he would need to wait for the right moment. This could also teach him to be more observant of his surroundings and the behavior of pokemon. And his bond with his existing pokemon could also strengthen as they work together to catch the rare one.
Ash could become more responsible as he has a unique starter that might need special care or training. He'd have to learn new ways to bond with it, which would make him more empathetic towards Pokemon in general. For example, if the starter is shy, Ash would have to be patient and gentle, thus developing his patience.
In these fanfictions, Ash often becomes more confident. Since he is the chosen one, he has a greater sense of purpose. For example, he may be more decisive in battles, quickly choosing the right Pokemon to send out.